The present invention relates to tangential servo control signal generating devices for recorded data reproducing devices. More particularly, the invention relates to a so-called "tangential servo control signal generating device" for controlling the movement of a pick-up in the tangential direction of a video disc in a data reading device.
Video data or the like is stored or recorded on the recording surface of a video disc by arranging minute pits in the form of spiral tracks with, for example, the length or depth of the pits varied according to the video signals recorded. Data recorded on a video disc is reproduced optically as follows. A pickup light beam is focused on the video track while the disc is rotated at a predetermined fixed speed so that the variations in intensity of light reflected by or passing through the disc are converted into electrical video signals.
In order to reproduce pictures satisfactorily with such a recorded data reproducing device, the provision of various servo devices is essential. One of the servo devices is a so-called "tangential servo control device" which is provided to correct the time axis of a regenerative signal. The tangential servo control device operates, for instance, as follows. With one prior art technique, the regenerative color burst signal in a regenerative signal and a sub-carrier signal (3.58 MHz) provided separately as a reference signal are subjected to phase comparison. A phase difference signal obtained as a result of the phase comparison is employed as a tangential servo control signal to control the rotation of a tangential mirror to thereby control the position of the pickup light spot in the tangential direction of the track.
The color burst signal is a signal which, as well known in the art, is obtained by generating 8 to 11 cycles of a signal having the same frequency (3.58 MHz) and phase as the sub-carrier on the "back porch" of the horizontal synchronizing signal. The color burst signal is generated once for every occurrence of the horizontal synchronizing signal, that is, it is an intermittent signal. If a relative position of the light spot in the tangential direction is shifted for some reason while the phase of the burst signal is shifted by an integer multiple 360.degree., the shift cannot be detected and the phase error is locked in and, accordingly, the phase errors disadvantaneously are accumulated.
In a second prior art technique, the "zero crossing point" of the regenerative color burst signal, which is delayed by a predetermined time from the regenerative horizontal synchronizing signal, is detected to provide a timing detection signal, the timing detection signal and a reference signal synchronous with a separately provided horizontal synchronizing signal are subjected to phase comparison, and the resultant phase difference signal is employed as a tangential servo control signal.
With this technique, if for some reason the phase of the regenerative color burst signal is shifted by 180.degree., it is impossible to detect the phase shift. When performing a "jump" operation, for example, for double-speed reproduction, the regenerative signal is different in frame and its regenerative color burst signal is shifted by 180.degree. in phase, that is, inverted. As the 180.degree. phase shift cannot be detected, the 180.degree. phase shift is locked in. Accordingly, in order to make the phase of the regenerative color burst signal continuous, disadvantageously it is necessary to provide an additional correction circuit.
In general, in the NTSC television system, the sub-carrier frequency (3.58 MHz) is an odd integer multiple of half of the horizontal synchronizing signal frequency. The color burst signal is superposed on the "back porch" of the horizontal synchronizing signal for several cycles with its phase difference with respect to the horizontal synchronizing signal being shifted by 180.degree. for every period of the horizontal synchronizing signal. In practice, the color burst signal is provided by a circuit in which the sub-carrier, which is a continuous sine wave, is superposed on a video compound signal by gating the sub-carrier with a burst flag signal for a predetermined period, usually 8 to 11 cycles. Therefore, the phase of the color burst signal is continuous.
In view of the foregoing, an object of the present invention is to eliminate the above-described difficulties accompanying a conventional system. More specifically, an object of the invention is to provide a tangential servo control signal generating device of simple construction in which, even if the phase of a color burst signal is shifted, the phase is corrected satisfactorily so that a color burst signal having a correct, continuous phase is provided at all times.
It is a further object of the invention to provide a tangential servo signal generating device for a recorded data reproducing device in which accumulation in one direction of a tangential error caused by large variations in the phase of the burst signal at the time of jump are prevented thereby to minimize the burst detection error.